The overall objective of this research project is to continue and expand our comparative studies of molecular regulatory mechanisms of muscular contraction. Regulatory proteins which bind calcium and confer a calcium-dependency to actomyosin interactions with ATP are located on either the actin-containing thin filaments, the myosin-containing thick filaments, or on both. Muscles in most invertebrates so far studied are calcium-regulated via a dual control on actin and myosin. However, thin filament-linked regulation has not been demonstrated conclusively in molluscs, the most extensively studied invertebrate phyla, although myosin-regulation is well documented. Conversely, myosin-regulation has not been demonstrated convincingly in vertebrate striated muscle in which the thin filament type of regulation is well established. The aim of our present investigations is to study various molluscan muscles to determine if indeed they have a dual system of calcium regulation.